1. FIELD OF THE INVENTION
The invention relates generally to electrical connectors, and more specifically, to electrical connectors of the feed-thru type used to provide an electrical connection through the wall of a pressurized vessel.
2. DESCRIPTION OF THE PRIOR ART
The problem of making an electrical connection through the wall or bulkhead of a pressurized vessel is, of course, a very old one. Some approaches which have been employed in the past include incapsulation where the feed-thru pin or conductor or terminal is held in position and sealed to its housing or support structure through the use of a suitable incapsulant such as an epoxide, a thermoset plastic material, a thermoplastic material or a silicone sealer or the like. Selection of sealing materials in this incapsulation approach depends upon environmental conditions and extremes to be encountered and also on the magnitude of the pressure differential which the feed-thru device must resist.
The employment of glass-to-metal seal technology has provided a basis for the feeding through of a pin or terminal. In such arrangements, the pin or terminal is held in position and/or sealed to its housing or support structure by the use of a glass compression seal or a mismatched glass-to-metal seal of the "housekeeper" type.
Ceramic-to-metal seals have also been employed for holding the feed-thru pin or terminal in position and/or sealing it to its housing or support structure. Metalized ceramics and ceramic-to-metal transition pieces permit the attachment of metal pieces thereto by brazing or hard soldering methods. Such ceramic insulating materials with metalized areas thereon have been known and used in the vacuum capacitor and circuit interrupter arts for many years.
The incapsulation concept first recited above suffers from poor reliability due to the difficulty of maintaining long-term "wetting" of the incapsulant to the structural members under high temperature and thermal shock conditions, especially in the presence of various fluids such as water, steam and radioactive liquids and gases. Moreover, incapsulants of the types described tend to age rapidly under the conditions imposed.
Glass-to-metal seals may be basically of two types, i.e., compression seals and "housekeeper" type. To keep the seal in a proper state of compression throughout wide temperature and pressure changes in the presence of fluids (including the aforementioned water, steam, and radio-active fluids) is a very difficult one. Still further, there is a tendency of glasses used for compression seals to have poor resistance to erosion caused by water and steam, etc. Then, too, there is the fact that the compression seal glass material has relatively poor structural strength and very low ductility and malleability which reduce its ability to withstand forces generated in handling and the aforementioned hostile environments on the high pressure side of feed-thru.
In the so-called "housekeeper" concept, the physical size of the necessary glass-to-metal seal subassembly makes it very difficult to construct a connector assembly of small size. The temperatures and pressures experienced in the very rigorous service conditions of water, steam and radio-active fluids under pressure and at relatively high temperature exceed the practical strength capabilities of the metal portion of the seal, be it copper, nickel-iron, or kovar. In high temperature aqueous environments, glasses used for the "housekeeper" type seal are quite susceptible to erosion. Finally, in respect to glass seals, the inherent restriction on assembly techniques imposed by the relatively low temperature capability of the metal-to-glass subassembly tend to rule out such processing steps as furnace brazing and the like.
While ceramic materials are inherently very desirable, their prior art use (as metalized ceramic) require that thin metal sections join the ceramic to the housing or support structure, and these parts are generally incapable of withstanding the large axial force generated by use of the connector in the high pressure, high temperature environment. It has proven to be very difficult to design and construct a flexible ceramic-to-metal seal which is functional and yet does not tend to separate radially from either ceramic or its support mechanism under the action of the high pressure differential and expansion forces introduced by high temperature in service.
Some prior art United States patents dealing with various aspects of the general problem include U.S. Pat. Nos. 3,455,708; 3,660,593; 3,685,005; 3,735,024; and 3,850,501.
U.S. Pat. No. 3,455,708 deals with ceramic material for use in devices of the type to which the present invention applies. The reference shows a conductive pin passing through a ceramic insulating block and is sealed thereto, whereas the ceramic block itself is sealed within an aluminum shell or body. The device basically makes no allowance for repeated coefficient of expansion differential nor would it be expected to perform satisfactorily in a steam environment due to the glass frit (silicon dioxide binder) being subject to erosion with consequent void and leak formation. The device is also subject to fatigue induced by thermal cycling and the design is basically limited to small pin diameters.
In respect to the devices described in other aforementioned U.S. patents, many or all of the aforementioned comments apply.
The manner in which the present invention advances the state of the hermetic electrical feed-thru art, by providing a connector greatly superior to those of the prior art in respect to resistance to high pressure, high temperature, and other adverse environmental factors over the long term, will be understood as this description proceeds.